1. Field of the Invention
The present invention relates to the fields of retinoid X receptor .gamma. (RXR.gamma.) biology and transgenic mice. Specifically, the present invention relates to mice which are deficient in the normal expression of the gene encoding the RXR.gamma. receptor, to mice heterozygous for such deficiency, to cell lines, preferably pluripotent or totipotent cell lines, which are heterozygous or homozygous for such deficiency, and to methods of using said mice or said cell lines to identify agonists and antagonists of the RXR.gamma. receptor.
2. Description of the Related Art
Retinoids
A number of studies have demonstrated that retinoids (vitamin A derivatives) are essential for normal growth, vision, tissue homeostasis, reproduction and overall survival (for reviews and references, See Sporn et al., The Retinoids, Vols. 1 and 2, Sporn et al., eds., Academic Press, Orlando, Fla. (1984)). Retinoids are also apparently crucial during embryogenesis, since offspring of dams with vitamin A deficiency (VAD) exhibit a number of developmental defects (Wilson, J. G., et al., Am. J. Anat. 92:189-217 (1953)). With the exceptions of those on vision (Wald, G., et al., Science 162:230-239 (1968)) and spermatogenesis in mammals (van Pelt, H. M. M., and De Rooij, D. G., Endocrinology 128:697-704 (1991)), most of the effects generated by VAD in animals and their fetuses can be prevented and/or reversed by retinoic acid (RA) administration (Wilson, J. G., et al, Am. J Anat. 92:189-217 (1953); Thompson et al., Proc. Royal Soc. 159:510-535 (1964)). The dramatic teratogenic effects of maternal RA administration on mammalian embryos (Shenefelt, R. E., Teratology 5, 103-108 (1972); Kessel, M., Development 115:487-501 (1992); Creech Kraft, J., In Retinoids in Normal Development and Teratogenesis, G. M. Morriss-Kay, ed., Oxford University Press, Oxford, UK, pp.267-280 (1992)), and the marked effects of topical administration of retinoids on embryonic development of vertebrates and limb regeneration in amphibians (Mohanty-Hejmadi et al., Nature 355:352-353 (1992); Tabin, C. J., Cell 66:199-217 (1991)), have contributed to the notion that RA may have critical roles in morphogenesis and organogenesis.
Retinoid Receptors
Except for those involved in visual perception (Wald, G. et al., Science 162:230-239 (1968)), the molecular mechanisms underlying the highly diverse effects of retinoids have until recently remained obscure. The discovery of nuclear receptors for RA (Petkovich et al., Nature 330:444-450 (1987); Giguere et al., Nature 330:624-629 (1987)) has greatly advanced the understanding of how the retinoids may exert their pleiotropic effects (Leid et al., TIBS 1 7:427-433 (1992); Linney, E., Current Topics in Dev. Biol. 27:309-350 (1992)). It is thought that the effects of the RA signal are mediated through two families of receptors--the RAR family and the RXR family--which belong to the superfamily of ligand-inducible transcriptional regulatory factors that include steroid/thyroid hormone and vitamin D3 receptors (for reviews see Leid et al., TIBS 17:427-433 (1992); Chambon, P., Semin. Cell Biol. 5:115-125 (1994); Chambon, P., FASEB J. 10:940-954 (1996); Giguere, V., Endocrinol. Rev. 15:61-79 (1994); Mangelsdorf, D. J., and Evans, R. M., Cell 83:841-850 (1995); Gronemeyer, H., and Laudet, V., Protein Profile 2:1173-1236 (1995)).
RAR Receptors
Receptors belonging to the RAR family (RAR.alpha., .beta. and .gamma. and their isoforms) are activated by both all-trans- and 9-cis-RA (Leid et al., TIBS 17:427-433 (1992); Chambon, P., Semin. Cell Biol. 5:115-125 (1994); Dolle, P., et al., Mech. Dev. 45:91-104(1994)). Within a given species, the DNA binding (C) and the ligand binding (E) domains of the three RAR types are highly similar, whereas the C-terminal domain F and the middle domain D exhibit no or little similarity. The amino acid sequences of the three RAR types are also notably different in their B regions, and their main isoforms (.alpha.1 and .alpha.2, .beta.1 to .beta.4, and .gamma.1 and .gamma.2) further differ in their N-terminal A regions (Leid et al., TIBS 17:427-433 (1992)). Amino acid sequence comparisons have revealed that the interspecies conservation of a given RAR type is greater than the similarity found between the three RAR types within a given species (Leid et al., TIBS 17:427-433 (1992)). This interspecies conservation is particularly striking in the N-terminal A regions of the various RAR.alpha., .beta. and .gamma. isoforms, whose A region amino acid sequences are quite divergent. Taken together with the distinct spatio-temporal expression patterns observed for the transcripts of each RAR and RXR type in the developing embryo and in various adult mouse tissues (Zelent, A., et al., Nature 339:714-717 (1989); Dolle, P., et al., Nature 342:702-705 (1989); Dolle et al., Development 110:1133-1151 (1990); Ruberte et al., Development 108:213-222 (1990); Ruberte et al., Development 111:45-60 (1991); Mangelsdorfet al., Genes & Dev. 6:329-344 (1992)), this interspecies conservation has suggested that each RAR type (and isoform) may perform unique functions. This hypothesis is further supported by the finding that the various RAR isoforms contain two transcriptional activation functions (AFs) located in the N-terminal A/B region (AF-1) and in the C-terminal E region (AF-2), which can synergistically, and to some extent differentially, activate various RA-responsive promoters (Leid et al., TIBS 17:427-433 (1992); Nagpal, S., et al., Cell 70:1007-1019 (1992); Nagpal, S., et al., EMBO J 12:2349-2360 (1993)).
RXR Receptors
Unlike the RARs, members of the retinoid X receptor family (RXR.alpha., .beta. and .gamma.) are activated exclusively by 9-cis-RA (Chambon, P., Semin. Cell Biol. 5:115-125 (1994); Dolle, P., et al., Mech. Dev. 45:91-104 (1994); Linney, E., Current Topics in Dev. Biol. 27:309-350 (1992); Leid et al., TIBS 17:427-433 (1992); Kastner et al., in Vitamin A in Health and Disease, R. Blomhoff, ed., Marcel Dekker, New York (1993)). However, the RXRs characterized to date are similar to the RARs in that the different RXR types also differ markedly in their N-terminal A/B regions (Leid et al., TIBS 17:427-433 (1992); Leid et al., Cell 68:377-395 (1992); Mangelsdorf et al., Genes and Dev. 6:329-344 (1992)), and contain the same transcriptional activation functions in their N-terminal A/B region and C-terminal E region (Leid et al., TIBS 17:427-433 (1992); Nagpal, S., et al., Cell 70:1007-1019 (1992); Nagpal, S., et al., EMBO J 12:2349-2360 (1993)).
It is currently unclear whether all the molecular properties of RXRs characterized in vitro are relevant for their physiological functions in vivo. In particular, it is unknown under what conditions these receptors act as 9-cis-RA-dependent transcriptional regulators (Chambon, P., Semin. Cell Biol. 5:115-125 (1994)). The knock-outs of RXR.alpha. and RXR.beta. in the mouse have provided some insight into the physiological functions of these receptors. For example, the ocular and cardiac malformations observed in RXR.alpha..sup.-/- fetuses (Kastner, P., et al., Cell 78:987-1003 (1994); Sucov, H. M., et al., Genes & Devel. 8:1007-1018 (1994)) are similar to those found in the fetal VAD syndrome, thus suggesting an important function of RXR.alpha. in the transduction of a retinoid signal during development. The involvement of RXRs in retinoid signaling is further supported by studies of compound RXR.alpha./RAR mutants, which reveal defects that are either absent or less severe in the single mutants (Kastner, P., et al., Cell 78:987-1003 (1994); Kastner, P., et al., Cell 83:859-869 (1995)). Moreover, it has been shown that activation of RA-responsive promoters likely occurs through RAR:RXR heterodimers rather than through homodimers (Yu, V. C. et al., Cell 67:1251-1266 (1991); Leid et al., Cell 68:377-395 (1992b); Durand et al., Cell 71:73-85 (1992); Nagpal et al., Cell 70:1007-1019 (1992); Zhang, X. K., et al., Nature 355, 441-446 (1992); Kliewer et al., Nature 355:446-449 (1992); Bugge et al., EMBO J. 11:1409-1418 (1992); Marks et al., EMBO J. 11:1419-1435 (1992); Yu, V. C. et al., Cur. Op. Biotech. 3:597-602 (1992); Leid et al., TIBS 17:427-433 (1992); Laudet and Stehelin, Curr. Biol. 2:293-295 (1992); Green, S., Nature 361:590-591 (1993)). These results strongly suggest that RAR/RXR heterodimers are indeed functional units that transduce the RA signal in vivo, although it is unclear whether all of the suggested heterodimeric combinations occur in vivo (Chambon, P., Semin. Cell Biol. 5:115-125 (1994)). Thus, the basis for the highly pleiotropic effect of retinoids may reside, at least in part, in the control of different subsets of retinoid-responsive promoters by cell-specifically expressed heterodimeric combinations of RAR:RXR types (and isoforms), whose activity may be in turn regulated by cell-specific levels of all-trans- and 9-cis-RA (Leid et al., TIBS 17:427-433 (1992)).
The RXR receptors may also be involved in RA-independent signaling. For example, the observation of aberrant lipid metabolism in the Sertoli cells of RXR.beta..sup.-/- mutant animals suggests that functional interactions may also occur between RXR.beta. and the peroxisomal proliferator-activated receptor signaling pathway (WO 94/26100; Kastner, P., et al., Genes & Devel. 10:80-92 (1996)).
RXR.gamma.
While RXR.alpha. and RXR.beta. have a widespread (possibly ubiquitous) expression pattern during mouse development and in the adult animal (Mangelsdorf, D. J., et al., Genes & Devel. 6:329-344 (1992); Dolle, P., et al., Mech. Devel. 45:91-104 (1994); Nagata, T., et al., Gene 142:183-189 (1994)), RXR.gamma. transcripts appear to have a more restricted distribution. In the embryo, RXR.gamma. is mainly expressed in developing skeletal muscles where its expression persists throughout life. RXR.gamma. is also expressed in the heart (after birth), sensory epithelia of the visual and auditory systems, specific structures of the central nervous system, and in tissues involved in thyroid hormone homeostasis, e.g., the thyroid gland and thyrotrope cells in the pituitary (Mangelsdorf, D. J., et al., Genes & Devel. 6:329-344 (1992); Dolle, P., et al., Mech. Devel. 45:91-104 (1994); Sugawara, A., et al., Endocrinology 136:1766-1774 (1995); Liu, Q., and Linney, E., Mol. Endocrinol. 7:651-658 (1993)).
It is yet unclear, however, whether this restricted expression of RXR.gamma. corresponds to specific functions in vivo. To address this question, it would be of great importance to be able to establish a living model wherein the role of the RXR.gamma. receptor could be studied in a definitive manner.
The mouse is the model of preference in the study of the mammalian genetic system, and a great deal of research has been performed to map the murine genome. Accordingly, it is an object of the present invention to generate strains of mice which do not express, or express at undetectable levels, the RXR.gamma. receptor. Such mice would be of great value for a better understanding of the role of RXR.gamma. because such animals, and cell lines leading to or derived from such animals, would allow direct testing of the function of specific genes, either deleted or reintroduced by transgenesis, and would serve as an assay system to identify compounds which act as antagonists or agonists of the RXR.gamma. receptor.